Conventionally, a structure which comprises a tubular lamp as a light source located at the focal point of a paraboloid of revolution type of reflector and an opalescent diffusing plate located above the lamp, is generally used. Such a device is improved by adjustment of the shape of the reflector or of the diffusion coefficient of the diffusing plate.
There exists a device having a combination of a tubular lamp and a light guide with a shape which is simulated by an approximation of a point-like source of light and is formed in the shape of the approximated curve so that lights in a direction are emitted, a device having a light guide the thickness of which varies along the advance direction of light, a device using a lenticular member having angles of prisms which vary according to the distance from a light source, and a device having a combination of these elements.
Recently, surface light source elements have been used as liquid crystal display elements. However, the display device, which uses a conventional surface light source element to obtain a high quality display, requires a thick surface light source element. In particular, a surface light source element for a large display of 10 to 12 inches requires a thickness of 20 to 30 mm. Therefore, it is impossible to function as a thin surface light source element.
Various edge-light types of surface light source devices have been proposed. Each of the devices has a transparent light guide comprising a plate-shape transparent material such as an acrylic resin. Light incident on an edge portion of the transparent light guide is then emitted from the upper or lower surface of the light guide. However, such a large liquid crystal display device with a size of 10 to 12 inches results in a display with darkness corresponding to the distance from the light source or an uneven display, so that a display of high quality cannot often be obtained.
In order to solve this problem, a light guide having a thickness which varies according to the distance from a lamp, or a member which varies light path geometrically, is provided. However, such a member requires a precise processing to form a specific shape, and high processing costs. Furthermore, the member provides a low utilization rate of light.
Recently, Japanese Patent Application Laid-open No. (Tokukai-Hei) 1-245220 discloses a surface light source element which is an edge-light type. The surface light source element has a light guide with a layer of a light diffusing material which is applied or adhered on the surface thereof opposed to a light emitting surface or on a light diffusing surface of a provided member. The density of the provided light diffusing material increases as the distance from the light incident portion increases. Japanese Patent Application Laid-open No. (Tokukai-Hei) 1-107406 discloses a surface illuminating device which can uniformly illuminate the whole surface of a light diffusing plate. The device has a plurality of piled transparent plates, each of which has a different pattern of fine dots (light diffusing material) thereon.
Since a non-light-transmissive inorganic material (e.g., white pigment such as titanium oxide or barium sulfate) is used, a loss of light occurs to lower the luminance of the emitted light.
One has reported a light diffusing device which comprises a light guide, a diffusing layer provided on the light guide through a layer having a middle characteristic between those of the light guide and the diffusing layer, and a light regulation member for obtaining a uniform emitting light on the diffusing layer, in Japanese Utility Model Application Laid-open No. (Jitsukai-Sho) 61-171001 and in U.S. Pat. No. 4,729,068.
The inventors have proposed a surface light source element which is an edge-light type, as described in Japanese Patent Application Laid-open Nos. (Tokukai-Hei) 1-244490 and 1-252933. The element has a lens-like surface or a satin finished surface provided on at least one of a light emitting surface of a light guide and the opposed surface thereof, and a light regulation member having a light reflecting pattern which corresponds to the reciprocal of an emitting light distribution and a light diffusing plate provided on the emitting surface of the light guide.
The light diffusing device and the surface light source element using the light regulation member show excellent effects regarding the uniformity of the emitted light. The device and the element, however, cannot reuse the light which is reflected by the emitting light regulating member, so that the luminance of the emitted light lowers near the minimum value of the luminance before regulation.
The object of the present invention is to provide a very thin type of surface light source element which can supply emitted light with a uniform and high luminance.